This invention generally concerns a process for removing residual acid from moist halogenated polymer resins during drying thereof. This invention more particularly concerns use of an inert drying gas to remove residual acid from moist halogenated polymer resins during drying thereof.
Polyolefins, e.g. polyethylene, are generally halogenated while in the form of finely-divided particles. One process provides for halogenation of the particles while they are suspended in an aqueous medium. A similar process provides for halogenation of the particles while they are suspended in a halogen-resistant solvent system which does not appreciably dissolve the resin. Polyolefin resins may also be halogenated while dissolved in a solvent system. These processes generate a considerable amount of acid, e.g. hydrochloric acid if the halogen is chlorine. The acid is dispersed throughout the aqueous medium or solvent system as well as within and on the surface of the particles. The acid must be removed or neutralized if the halogenated polymer is to be useful for thermoplastic and elastomer applications.
Batch or continuous water washing to remove residual acid gives rise to polymer particle agglomeration as noted by Ohorodnik et al. in U.S. Pat. No. 4,440,925. Staged water washing is time consuming. It is costly because of needs for process water preparation and waste water cleanup. It also requires the use of additives like surfactants or low boiling alcohols to enhance removal of acid from within the polymer particles. If further requires the use of acid-resistant equipment to remove aqueous acid from the polymer between wash cycles. Neutralization of the acid by adding caustic soda or some other alkaline or base material leads to the formation of acid salts within and on the surface of polymer particles. The presence of such salts results in poor electrical properties and high water swell.
Halogenated polymer resins separated from the suspension medium, either without a water wash or with limited water washing, have high residual acid contents. Drying such resins with heated air leads to product quality problems, particularly in the areas of product color and thermal stability.
Nitrogen purging of chlorinated polymers prepared by solution chlorination is described in U.S. Pat. Nos. 4,381,392, 4,358,402, 4,122,249, Japanese Patent publication No. 5,408,139 and Belgian Patent No. 850,493. The purpose of the purge is to remove excess chlorine. Chlorinated hydrocarbon solvents can be removed by steam stripping or nitrogen purging.
It would be desirable if there were available a process for removing residual acid from halogenated polymer resins which did not include one or more water wash steps or cycles. It would also be desirable if such a process did not require the use of additives such as surfactants to aid in removal of residual acid. It would further be desirable if such a process did not require an acid neutralization step.